This invention relates to improvements in communications systems and, more particularly, to a distortion correction apparatus with application to an optical communication system in which video information is transmitted over an optical link to a receiver.
In a common type of television communication over an optical communication system, such as one employing a fiber optical link, an amplitude-modulated television video signal is utilized to drive a light-emitting diode. The light emitted from the diode, which contains the video information, is carried over the optical link and then recovered at a receiver.
In present and forseeable state of the art, a light-emitting diode is a necessary important component of the transmitter in an optical communication system such as a fiber optics communication system. Light-emitting diodes, however, have been found to introduce troublesome distortions to signals applied thereto. When a video signal is applied to a light-emitting diode in the transmitter of an optical transmission system, these distortions result in degradation of the video when it is recovered at the receiver end of the optical communications system. Distortions introduced by light-emitting diodes involve, inter alia, the following phenomena: The output of a light-emitting diode is temperature sensitive. The light-emitting diode junction temperature changes with video content as the current through the diode causes it to heat up. Therefore, the light output changes with picture brightness or "A.P.L", which stands for "average picture level". In a television video signal the sync tip time has a different A.P.L than the picture, so the sync tip is also distorted. The vertical sync tip may be substantially distorted, but the horizontal sync tip is generally not distorted noticeably, since the heating time constant of the light-emitting diode junction is longer than the horizontal sync time.
The described types of distortions generally decrease light output with increasing temperature. Accordingly, one might normally assume that such distortions could not be readily corrected for by using a passive circuit which does not provide gain in the correction.
It is an object of the present invention to correct for distortions caused by light-emitting diodes in the described type of optical communication system, and to do so with circuitry that is not unduly complex and expensive, and which can be passive.